"The American side ... should try not to spout nonsense," Khamenei said in a speech. "Saying things like 'we won’t allow Iran to enrich uranium' is way out of line," he added. "We do not think (the talks) would yield results now."
Within hours of Khamenei's speech, the Iranian rial weakened by over 3 percent, and the main index of the Tehran Stock Exchange dropped by around 2 percent.
Some supporters of a hard Iranian line said a US moratorium on Iranian enrichment, which Tehran maintains is its national right, would scupper the talks.
“The Leader stood up to America's excesses. The US will be directly responsible if the negotiations bear no result. They have no right not to recognize (Iran's) right to enrichment,” hardline cleric Ehsan Ebadi posted on X.
But the sharp speech also raised alarm about the stakes of the talks' collapse.
“The failure of the negotiations is definitely a loss for Iran, not the US. Iran should not let the negotiations fail,” Iranian teacher Ali Ghiasi-Farahani posted on X.